As a photovoltaic power generation panel system, there has been frequently used a flat plate system in which solar cells generally called cells are laid over a sunlight receiving surface. Since the solar cells to be laid over according to the light receiving area are expensive, it has been difficult to lower the cost of the photovoltaic power generation panel. In view of this, a concentrating type photovoltaic power generation panel has been developed in which the light at the sunlight receiving surface is concentrated (condensed) onto small-diameter solar cells. In the concentrating type photovoltaic power generation panel, light is concentrated by a factor of several hundreds (see, for example, Patent Document 1: JP-A 2003-258291), whereby the number of solar cells to be used can be remarkably reduced as compared with the flat plate system. However, the concentrating type photovoltaic power generation panel in which light is concentrated by a factor of several hundreds has a problem in that biaxial tracking for orienting the sunlight receiving surface to the sunlight direction is needed. Further, the trestle for the tracking has to be robust, taking into consideration the outdoor use of the panel; the installation cost of the trestle would cancel out the initial cost-basis merit of a marked reduction in the number of expensive solar cells to be used. For this reason, a system in which monoaxial tracking is adopted or no tracking is conducted, for the purpose of facilitating the sunlight tracking and reducing the cost, is proposed at present. In this system, a resin-made Fresnel lens or prism type sheet is disposed for the condenser lens so that externally coming incident light is condensed into the direction normal to the light receiving surface, then the condensed light is reflected, transmitted or refracted into a predetermined direction, and the distributed light is further guided by total reflection to finally enhance the incident light energy density (see, for example, Patent Document 2: JP-A 2009-229581).
In order that the photovoltaic power generation panel becomes wide spread as a measure for solving the global warming problem, a cost reduction at such a level as to compete with the power cost in the network commercial power system should be achieved. The cost for the photovoltaic power generation panel is said to be 100 yen/Wp, and, accordingly, there is a demand for excellent durability as well as low material cost and low processing cost for the shaping of the sunlight concentrating (condensing) section such as Fresnel lens or prism lens.
As a material for the sunlight concentrating section such as the resin-made Fresnel lens or prism lens, in general, acrylic resin, polymethyl methacrylate resin and polycarbonate resin are used. In the concentrating solar cell panel required to show long-term durability of more than twenty years under sunlight, however, silicone resins higher than these general-purpose resins in resistance to UV rays is suitably used. In the case of processing the shape of the sunlight concentrating section such as Fresnel lens or prism lens, however, a low-productivity working process is adopted in which a resin is injected into a mold by injection molding and the product is taken out after the molding, like in the cases of the acrylic resin for general-purpose use. This results in a raised production cost. For this reason, it has been difficult to put the simple, light-weight concentrating photovoltaic power generation panel to practical use by using the sunlight concentrating section such as resin-made Fresnel lens or prism lens.
On the other hand, attendant on the trend toward higher functions of cellphones, in regard of illuminating backlight devices there is an increasing demand for reduced thickness, reduced weight, enhanced luminance and reduced cost. In response to this demand, there has been proposed an illuminating backlight device using an optical waveguide plate (light guide plate), which is also adopted in liquid crystal displays (LCDs). This device includes an LED light source, and an optical waveguide plate by which light is guided from an end face facing the light source into the inside and is diffused, the diffused light is reflected by a reflective surface disposed at a key part to be illuminated, and the reflected light is projected to the exterior (see Patent Document 3: JP-A 2003-59321). This device is characterized in that LEDs are arranged in a lateral direction, whereby a reduction in thickness is achieved, and the whole surface of the keypad can be illuminated evenly and in high luminance even if the number of LEDs is reduced, as compared with the conventional type in which LEDs are arranged on the back side. Besides, since power consumption is also lowered, reductions in the size and weight of battery are advanced, contributing to a reduction in cost.
The optical waveguide plate for an LCD backlight device is required only to be good in optical characteristics. In contrast, the optical waveguide plate for a keypad illuminating backlight device is required not only to be high in transparency but also to be low in elasticity and in temperature dependence of elasticity, and to be capable of being formed into a smooth-surfaced film with a small and uniform thickness, because it also functions to transmit a key input displacement (click) to a switching element. In general, acrylic resin and polycarbonate resin are high in transparency and good in film forming properties. However, these resins show temperature dependency of elasticity, particularly embrittlement in low temperature regions, so that such a resin may be cracked at the time of key input. In view of this, highly transparent silicone resins applied also as optical material such as potting material for LEDs have been paid attention to (see Patent Document 4: JP-A 2002-265787, Patent Document 5: JP-A 2006-202952, and Patent Document 6: JP-A 2006-342200) because of their excellent low-temperature properties. However, these materials are liquid type materials, so that the process of producing the optical waveguide plate therefrom is intricate, causing a rise in cost.